The present invention relates to dielectric ceramic compositions for microwave applications and, more particularly, to dielectric ceramic compositions for microwave devices such as, for example, dielectric resonators designed to be operated in a frequency range of microwaves.
The ceramic compositions for microwave resonators are required to have various electrical and physical properties such as, for example, high permittivity(.epsilon.), high quality factor(Q), small temperature coefficient of resonance frequency(.eta..sub.f), and good sintering characteristic.
As dielectric ceramic compositions for microwave applications, there are known those consisting essentially of MgTiO.sub.3 -CaTiO.sub.3, or MgTiO.sub.3 -CaTiO.sub.3 -La.sub.2 Ti.sub.3 O.sub.7 with 10 to 30 mol% of La.sub.2 Ti.sub.2 O.sub.7. Although the compositions of the MgTiO.sub.3 -CaTiO.sub.3 -La.sub.2 Ti.sub.3 O.sub.7 system exhibit high permittivity of about 30, they have a serious disadvantage that the Q is extremely low and less than 2000 at 7 GHz. One of the components, i.e., La.sub.2 Ti.sub.2 O.sub.7 per se exhibits extremely low Q of a space about 300 at 7 GHz so that the presence of a large amount of La.sub.2 Ti.sub.2 O.sub.7 causes the deterioration of the Q. It is therefore impossible with these compositions to produce dielectric resonating elements having desired characteristics required in microwave devices. The compositions of the MgTiO.sub.3 -CaTiO.sub.3 binary system have such favorable characteristics that (1) the permittivity is relatively high, (2) the Q at microwave frequencies is extremely high, and (3) the dielectric resonating elements, the temperature coefficient of resonance frequency of which is about 0 ppm/.degree.C., can be produced. Although these binary system compositions are useful as ceramic materials for microwave applications, they have the disadvantage that the sintering characteristic becomes worse as the resonating element size becomes large and that they are poor in the linearity of the temperature characteristics.
It is therefore an object of the present invention to provide dielectric ceramic compositions for microwave applications with desired electrical and physical properties, which overcome the aforesaid disadvantages.
Another object of the present invention is to provide dielectric ceramic compositions which have a good sintering characteristic and exhibit relatively high permittivity, high Q, small temperature coefficient of resonance frequency, and good linearity of the temperature characteristics of electrical properties.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.
According to the present invention there is provided a dielectric ceramic composition for microwave applications consisting essentially of a sintered mixture represented by the general formula: EQU (1-x)MgTiO.sub.3 --x(Ca.sub.1-y Me.sub.y)TiO.sub.3
wherein Me is at least one rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and wherein x and y are molar fractions of respective components with values within the following respective ranges: 0.03.ltoreq.x.ltoreq.0.15, 0.001.ltoreq.y .ltoreq.0.06.
The above limitation on the molar fractions of the components is determined for the following reasons: If x is less than 0.03 or more than 0.15, an absolute value of the temperature coefficient of resonance frequency is considerably increased. If y is less than 0.001, the effect of the addition of the rare earth element to depress the grain growth is scarcely obtained; while with a large content of more than 0.06, the Q of the compositions having a MgTiO.sub.3 :CaTiO.sub.3 ratio in which the temperature coefficient of resonance frequency is 0 ppm/.degree.C. is extremely lowered.
According to the present invention, Me, i.e., at least one rare earth element selected from the group of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu is added to depress the grain growth, thereby improving the sintering characteristics of the composition. The effect of the above rare earth element on the MgTiO.sub.3 -CaTiO.sub.3 system ceramic compositions will become clear from the following description.